The present invention relates to a hydraulic system with a charge pump and a main pump, and more particularly to a hydraulic system in which a fixed displacement charge pump supplies fluid to a variable displacement main pump.
When an engine-driven fixed displacement charge pump is used to supply fluid to a variable displacement high pressure main pump, such as an axial piston pump, it is necessary to have the charge pump displacement large enough to satisfy the requirements of the main pump at low engine speeds. At high engine speeds, the output of the charge pump increases and provision must be made for excess flow not needed by the main pump. Typically, such excess flow is returned to sump via a relief valve. In applications where main pump flow is minimal much of the time, this technique circulates unneeded flow through the circuit and reservoir, contributing to aeration and power loss, or requires large line sizes to keep pressure drop low. It would be desirable to have such a system wherein a useful function is performed by the excess fluid flow and power loss is minimized.